Cathode ray beam control apparatus



March 3l, 195.9 M. M. ARMSTRONG ETAL 2,880,366

cATHoDE RAY BEAM vCONTROL APPARATUS Filed Aug. 21, 195e A HazSEP/1847019 00A/V.

CATHODE RAY BEAM CGN'IROL APPARATUS Merlyn M. Armstrong, Oak Park, andRichard G. OFallon, Westchester, lll., assignors to Motorola, Inc.,

Chicago, Ill., a corporation of Illinois Application August 21, 1956,serial No. 605,302

s claims. (ci. 315-27) The present invention relates to color televisionreceivers and more particularly to a system for controlling the rastersize produced by one beam of a tri-beam cathode ray tube as used in suchreceivers.

In the usual tri-beam cathode ray tube the beam sources are equallyspaced about the axis of the neck of the tube, and the three beams aredeflected simultane- -ously by a common line and field deflection systemin order that they scan the screen. In such a tube it is necessary thatthe three beams be converged at each aperture of a shadow mask disposedbetween the beam sources and screen so that they pass through theapertures to impinge upon their associated phosphor dots of the screenand that such beam convergence be maintained throughout the entirescanning cycle of the beams. However, since the electron beams arescanned through a rather wide angle and across a mask all parts of whichare not equidistant from the sources of the beams, the amount ofconvergence applied to the beams must be varied during the scanningoperation, and a dynamic convergence control must be used. This controlis synchronized with the line and lield sweep of the beams so that theyremain converged throughout the entire scanning operation.

Usually the tube is oriented so that the beam gun associated with theblue phosphor dots in each dot triad is on the vertical axis of thetube, with the red and green beam guns respectively spaced 120 aroundthe tube neck on each side of the blue beam gun. Then a iield producingdevice for dynamically converging each beam is mounted adjacent the pathof each of the beams. These iield producing devices also commonly haveprovision for convergence of the beams when undeilected, i.e. in staticcondition. However, the fields for both dynamic and static convergenceprovide beam control along radial lines spaced at 120 intervals aroundthe tube neck. Thus it may be visualized that two of the beams, inpractice the red and green beams, may be statically converged at kUnitedStates Patent C) the screen center by adjustment of the associated iieldv producing devices, but that such adjustment may not necessarilyconverge the remaining beam, the blue one,`

at the same point. This situation is remedied in the present state ofthe art by utilizing a blue beam lateral corrector magnet to providelateral or tangential shift of this beam at right angles to the eiect ofthe convergence control thereof. Of course, once static convergence isobtained, the dynamic convergence signals may be applied to effectalignment of the beams in all scanned positions to provide threedifferent rasters which appear superimposed. The above describedconvergence control is commonly applied to the beams before they aredeected but it may be appreciated that deflection is applied to thebeams when they are equilaterally spaced from one another about the tubeaxis. Therefore, if the deflection field is not niformin its effect onall of the beams, there can be a; noticeable impairment of the compositeraster due to the dilerence in deection of one beam with respect toanother. The practical construction of cathode ray tubes tube.

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and deflection yokes presents some difliculties in this regard due tothe problem of maintaining design tolerances. Additionally, there can bevarious inter-actions among the components of a television receiverwhich also tend to adversely effect the desired registry of the rasters.As a still further problem in receiver construction, it has been foundthat an optimum deection field for the red and green beams to minimizewhat is known in the art as keystoning, or the effect of the red rasterbeing larger than the green raster on one side of the screen and thereverse situation on the other side of the screen, often tends to impairthe blue raster. This is because some tube and deflection yokecombinations produce minimum keystone effect when the horizontaldeflection field is somewhat stronger on either side of the exact centerof the tube neck. As previously pointed out, the blue'beam travels apath intermediate that of the red and green beams as far as thehorizontal deection field is concerned so that an optimum deilectionfield for the red and green beams may not be optimum for the blue beam.In most cases, the blue beam, in experiencing a weaker deflection field,tends to produce a smaller raster than both the red and green beams.However, in some receivers of practical construction it has been foundthat optimum adjustment of the red and green rasters is obtained whenthe blue raster is somewhat larger than the other two.

Accordingly, it is an object of this invention to provide easilyadjustable apparatus for a color television receiver for controlling thesize of the blue raster with respect to the red and green rasters.

Another object is to provide apparatus which is adjustable to compensatedynamically for dierent tri-beam cathode ray tube and deflection yokecombinations, as well as to compensate for the eects of certain types ofcorrections applied to one or more of the beams in the cathode ray tube.

Still another object of the invention is to provide a simple,inexpensive system for improving the color rendition in a colortelevision receiver.

A feature of the invention is the provision of a dynamically controlledfield producing device disposed adjacent the path of one beam in atri-beam cathode ray tube and oriented to produce a field forcontrolling the beam in the direction of the line sweep thereof toobtain dynamic variation of the raster size produced by the beam.

Another feature of the invention is the provision of a blue beam lateralcorrector in a tri-beam cathode ray tube including an adjustablepermanent magnet and an adjustable inductor associated therewith. Theinductor is energized by a sawtooth signal obtained from the line sweepsystem in order to effect selected dynamic varia-U tion of the scansionor lateral deflection of the beam.

Further objects, features and the attending advantages of the inventionwill be apparent upon consideration of the following description whentaken in conjunction with the accompanying drawing in which:

Fig. l is a block diagram of a color television receiver incorporatingthe invention;

Fig. 2 is a sectional view of the neck of a tri-beam cathode ray tube;

Fig. 3 is a perspective view of a portion of the beam controllingapparatus of the invention; and

Fig. 4 is a schematic diagram of a portion of the circuit of Fig. l.

In the preferred form of the invention there is provided a blue beamlateral corrector for a tri-beam cathode ray The usual orientation of atri-beam cathode ray tube establishes the blue -beam gun along avertical axis of the tub'e which positions this gun approximatelycentrally of the horizontal dimension of the tube screen. An adjustablepermanent magnet is supported adjacent the pre,q

convergence path of this beam tofurnish lateral shift thereof for staticconvergence purposes. Also included with the permanent magnet is atapped inductance coil -with itsvfield selectively additive orsubtractiveto that of thepermanent magnet lield. A switch associatedwith this coil provides adjustment of the field produced thereby andthis entire device is series coupled with the line sweep system and itsassociated deflection yoke of the television receiver. Accordingly,a'sawtooth current wave form is applied to the-inductance coil and thiseffects'a dynamic change in thebluebeam raster'size to assist vin theoptimum reproduction ofcolor images by the tube.

Referring now to the drawing, Fig. 1 shows a color televisionreceiverlOcoupled to an antenna 11. Receiver 110 ,provides signals derived from areceived television wave for the cathode ray Iimage reproducing device14 and these signals are applied to the equally'spaced electron .beamsources 17, 18 and 19. These beam sources are associated with vthe blue,red andgreen primary colors and produce electron beams 21, 22 and 23,respectively. :Anxaperturedshadow'mask 25 is disposed near the screen2f/fand `beams 21, 22 and 23 are converged in the individual aperturesof this mask during the scanning operation so that each beam impingesupon-its associated phosphor dots in each triad, such as triad 29. Thedifferent dots of each triad produce the colors red, blue and green whenimpinged by the beams. The triads then each become a colored element ofa complete image. It may be appreciated that the apparatus is showngreatly out of proportion to facilitate explanation.

A sound system 31 is also coupled to the receiver 10 and this systemapplies the derived audio to loudspeaker 33. The receiving systemfurther includes a synchronizing signal separator circuit 36 which,together with the vertical or field dellection circuit 38 and thehorizontal or line deflection circuit 39, comprises the sweep system ofthe receiver. Circuits 35 and 39 are coupled to the deflection yoke 41which is disposed on the neck of the cathode ray tube 14 so that signalsapplied to the yoke produce suitable line and field scanning of thebeams simultaneously across screen 27.

It should be apparent that as the electron beams lare scanned across thescreen they will not always converge in an aperture of mask 25 sincethe'mask and screen do not `have suicient curvature to coincide with thearc of the convergence point as the beams are scanned. The beam'sources17, 18 and 19 may be tilted somewhat so that the beams will be convergedat the center of screen 27, at least to the extent possible withinmanufacturing tolerances, but since convergence would not prevail atother than the center of the screen, the beams would ordinarily notimpinge their intended phosphor dots in all the triads. Accordingly, thepoint of convergence is continually varied during each line and iieldscanning cycle by means of a dynamic Vconvergence system 42. System 42is connected to line and field deflection circuits 39, 38 and iscontrolled by signals `from these circuits. The output of theconvergence system is applied to the dynamic convergence coils 45, 46and 47. These coils are mounted in proximity to the paths of the beamsfrom sources 17, 18 and 19, respectively so as to develop fields forcausing proper convergence of the beams. 'The line deflection circuit 39is also coupled to the blue beam control apparatus 50, the exactfunctioning of which will be explained in greater detail.

Fig. 2 vis a sectional representation of the neck 14a of the cathode raytube 14. It ymay be noted vthat convergence coil 4S is disposed alongthe vertical axis of the neck and that coils-46 and 47 arelpositioned onopposite sides of electrode 45 and at 120 therefrom. Each of these coilsis coupled to the dynamic convergence system 42 to be energized thereby.The coils further include separate pole vpieces between which aredisposedrespective Vpermanent magnets 45a, 46a and 47a. Theseniag-V netsare magnetized across their diameters so that rotation thereof will varya field set up in the pole pieces of the coils. The pole piecesarefurther magnetically coupled to internal pole pieces of the cathode raytube between which the electron beams travel. The internal pole piecesare designated 45b, 46b and 47b. The electron beams 21-23 are movedalong respective radial lines which are spaced apart by means of themagnetic fields established between the internal pole .pieces by coils.4S-47.

For static convergence of the beams, generallyeifected at vthe center ofthe screen, the permanent magnets 45a, 46a and 47a are adjusted.However, due to manufacturing tolerances or variations in a particularreceiver circuit, the crossing point or convergence point of the beams22, 23 may not lie on the vertical line along which the beam 21 may zbemade to travel by adjustment of magnet 45a. Accordingly, in colortelevision systems of this type, it has been common to provide a furtherpermanent magnet for lateral vcorrection of beam 21 that is tangentialmovement thereof with respect toa circle through the normallyequilaterally spaced beam paths. This correction would, of course, beapplied to move the beam horizontally or in the direction of the linesweep of the beams as scanned across the screen.

The blue beam correction apparatus 50 Ato accomplish this lateral shiftof the blue beam is shown in Fig. l3. This apparatus includes anon-magnetic clamping strap 60 which is fastened to an external magneticpole piece 62 forming a frame structure which may be securely locked onthe neck 14a of the cathode ray tube. This pole piece also includes anintegrally formed bridge 62a which extends outwardly from the tube neckand forms a supportfor the shaft 64. Shaft 64 is rotatably mountedin-bridge62a and carries a permanent magnet 66 which is magnetizedacross a diameter of shaft 64. The correction apparatus 50 is mounted onthe tube neck with the permanent magnet 66 near the path of the bluebeam 21 as shown in Figs. 1 and 3. The usual tri-beam cathode ray tubeincludes internal vblue lateral pole pieces 68 which provides a magneticflux path through the tube neck. This ux path is illustrated by thedotted lines 70 in Fig. 3 and is in part essentially perpendicular tothe beam 21. It should be apparent that lby rotation of shaft 64 eitherthe north or south poles of magnet 66 may be brought into proximity withthe beam 21 and that by such adjustment, lateral shift of this beam maybe effected. Such lateral adjustment of the blue beam together withadjustment permanent magnets 45a, 46a and 47a can provide completestatic convergence of the electron beams. It is preferable to space theends of bridge 62a considerably greater than the length of magnet 66 toreduce the tendency for ux shunting by the bridge and to increase thesensitivity of the dynamic control.

As previously mentioned, it has been found 'that due to differences invarious tube and deflection yoke combinations and some types ofcorrections applied to the red and green beams, as well as other randominteractions among the components of the television receiver, thehorizontal deflection of the blue beam may not be the same amount as thedeflection of the red and green beams. This results in a variation ofthe blue raster size as compared with the sizes of the red and greenrasters. In order to correct this diculty and to permit an adjustmentwhich compensates for its occurrence, the blue #beam lateral correctionapparatus also includes a provision for dynamically changing the size ofthe blue raster. This consists of a magnetic core 75 supported by andextending downwardly from the center of bridge 62a, and thereforedisposed along the vertical axis of the tube neck, and an inductancecoil 77 wound upon this core. The apparatus 50 preferably furtherincludesa the magnetic field produced bythe unit. It is conterm platedthat coil 77 be energized by a sawtooth current wave form at the linedeiiection frequency, which can be derived from circuit 39, in Order toprovide a variation of the scanning of the blue beam according to the'deected position of all of the beams to compensate for ithe abovedescribed deficiencies.

The circuit shown in Fig. 4 illustrates a manner of connecting theinductance coil 77 so as to provide the proper energization thereof. Itmay be seen that deilection yoke 41 consists of a pair of elddeiiectionV coils 84, 85 disposed on opposite sides of the tube neck 14aand series connected through the fixed portion of potentiometer 87.Similarly line deflection coils 89, 90 are supported along oppositesides of the tube neck and are series connected through a variableinductor 92. Deection signals at the eld frequency, ordinarily signalsof sawtooth current Wave form a 60 cycles per second, are applied to amovable arm of potentiometer 87 and the common interconnection of elddeflection coils 84, 85 by means of an output winding 95 ofverticaloutput transformer 97. Transformer 97 is included in the eld dellectioncircuit 38 and may be energized by means of circuits which are known inthe art. Similarly, a horizontal output transformer 99 is included inthe line dellection circuit 39 and may be energized by means of knowncircuits. An output winding 101 of transformer 99 has one side coupledto the common interconnection of line deflection coils 89 and 90 and afurther terminal coupled through the blue beam correction apparatus 50to a center tap point of variable inductor 92. Accordingly, currentsignals of sawtooth wave form, generally at 15.75 kc., are applied tothe line deflection coils so as to scan the electron beams in a,horizontal direction. Adjustment of potentiometer 87 and variableinductor 92 permits variation in the signal currents in the differentyoke coils and may be termedbalancing elements of the `deilectionsystem. Adjustment of inductor 92 provides a variation in the fieldsproduced by coils 89 and 90 to correct for red, green horizontal lineskewness, or divergence of the horizontal traces produced by these`beams. The balancing potentiometer 87 similarly allows regulation of theeld produced by coils 84 and 85 to correct for differences in the red,green vertical raster size, that is, the parallel but spaced tracesdeveloped by the red and green beams along the top or bottom sides ofscreen 27. Potentiometer 87 and variable inductor 92 may be convenientlymounted in a supporting device 105 (Fig. 1) which forms a part of theyoke structure 41. This balancing system is described more fully andclaimed in copending application of Richard G. OFallon, bearing SerialNo. 505,619, filed May 3, 1955, now U.S. Patent No. 2,825,846.

f It has also been found preferable to include in a television receiverof this type a provision for the correction of keystoning of the red andgreen rasters. This effect may be observed as an increase of the redraster size on` one side of the screen and a decrease in the sizethereof on the` other side of the screen. At the same time the greenraster size is decreased on the first side of the screen but increasedon the opposite side thereof. Such a keystone effect is the result ofVvariation in the yoke leld distribution from the design center and is,of course, due to the fact that the beam sources are spaced from oneanother and that as a practical matter the deflection yokes and cathoderay tubes of producion quality do not provide an optimum match for oneanother so as to produce equal elects on these spaced beams. A method ofcorrecting this keystone effect is to utilize small pieces of low lossmagnetic material which may be introduced disposed along the upper partof the tube neck and a similar adjustable slug 90a is shown disposedalong the lower part of all within the yoke eld. It is preferable toconstruct slugs 89a and 90a of low loss magnetic material and to havethem adjustable with respect to the yoke field as shown in Figs. l and4. These correctors are described and claimed in copending applicationof Gerald Caprio, Serial No. 607,498 filed August 31, 1956.

With a given tube and deilection yoke combination, it may be found thatoptimum keystone correction requires unequal or unbalanced adjustmentsof the slugs 89a and 90a in each coil window thus producing a red, greenhorizontal line skewness condition. As previously pointed out, this maybe corrected by adjustment of variable inductor 92. However, in makingthese beam corrections there may very well be a change in thedistribution of the yoke field so as to alter the size of the rasterproduced by the blue beam. For example, the above described correctionsmay produce a deflection yoke eld which is stronger on opposite sides ofthe vertical axis through the tube neck than it is exactly on this axis.Accordingly, the red and green beams would be deflected by a strongerline scanning field than would the blue beam, which is disposed on thevertical axis of the tube neck. It is also possible that the oppositecondition could exist wherein optimum control of the red and green beamis produced by a yoke eld which is stronger in the center than eitherside thereof. The blue beam lateral correction apparatus 50 provides adynamic correction of the blue beam raster size to account for this typeof variation in the yoke field distribution.

As may be seen in Fig. 4 the line deflection signals of sawtooth currentwave form are applied to the deflection yoke 41 through the inductancecoil 77. The magnetic core 75 of this coil is, as previously described,supported adjacent the permanent magnet 66 so that the field proatvarious positions of the yoke deflection eld to vary slightly the fieldas produced by one coil with respect duced by this coil is in additiveor in subtractive relation with the iield of the permanent magnet.

Various tap points of coil 77 are connected to the switch 80, which isof the shorting type so that its adjustment will not interrupt the linescanning signal. It is preferable to couple one terminal of outputwinding 101 of transformer 99 to a portion of coil 77 which isintermediate the end terminals thereof so that by connecting contacts ofswitch 80 to various points on coil 77, the influence on the blue beammay be such as to increase or decrease the raster size produced thereby.

The functioning of the switch may be understood by considering that asawtooth signal applied to coil 77 will be zero. and that it willincrease as the beam is scanned to either side of the center of thescreen so that the raster size produced by this beam is larger orsmaller than that produced by the other beams, depending on the settingof switch 80. Thus by adjustment of this switch a sawtooth signal at theline deflection frequency may add to or subtract from the eifect of theline deflection of the blue beam with respect to the deflection of allof the beams produced simultaneously by yoke 41.

The invention provides therefore, simple apparatus for a colortelevision receiver to control the image produced thereby. The apparatusis of comparatively inexpensive construction and permits an adjustmentwhich is highly desirable to correct for variations in tube and yokecombination as well as to compensate for other types of` correctionswhich may be applied to the beams of a tribeam cathode ray tube all forthe purpose of producing optimum registration of the rasters produced byeach of the beams.

"We claimt l. In a color television receiver including a tri-beamcathode ray tube having a neck section through which the beams passalong paths substantially equally spaced about an axis thereof and lineand eld sweep .systems which develop respective sawtooth signals fordeecting the beams to form rasters'associated with each beam,

assunse apparatusfor controlling the raster size produced by one beamwith respect to the raster sizes produced by the other beams, includingin combination, field producing means disposed in proximity to apredeflection path of saidone beam, said field producing means beingoriented to provide a field producing tangential displacement of sardone beam with respect to a circle passed through said beam paths in saidtube neck, and circuit means .for coupling the line sweep system tosaidv field producing, sa1d c1rcuit means supplying signals ofsubstantially sawtooth form for energizing said field producing meansthereby and dynamically controlling said raster size produced by saidone beam in accordance with line deflection signals. i

2. In a color television receiver including a tri-beam cathode ray tube,line and held sweep systems for deflecting the beams to producerespective rasters associated therewith, and a convergence controlsystem for dynamically controlling the beams according to the deflectedpositions thereof, apparatus for dynamically controlling the raster sizeproduced by one beam with respect to the raster sizes produced by theother beams, including in combination, an adjustable permanent magnetdisposed adjacent a preconvergence path of said one beam and oriented toproduce a field substantially perpe'ndicular to the direction of linedeflection, a tapped inductance coil supported adjacent saidv permanentniagnet and oriented to produce a field additive and subtractive withlthe field therefrom, a switch for selecting a tap'on said inductancecoil, and means for coupling said switch and' said inductance coil tothe line sweep system for energizing said inductance vcoil andcontrolling the size of the raster associated with said one beam inaccordance with line deflection signals.

3, In a color television receiver including a tri-beam cathode" raytube, line and field sweep systems for applying sawtooth signals to adeflection yoke for simultaneously deflecting the beams to producerespective rasters associated therewith, and a convergence controlsystem for dynamically controlling the beams according to the deflectedpositions thereof and wherein the line deflection of one beam, adaptedto travel a path intermediate the paths of the other beams with respectto the line sweep direction, may be different from the line deflectionof such other beams, apparatus for dynamically controlling the amount ofdeflection of said one fixed field which field shifts said one beam inthe direc-y tion of line deflection, a tapped inductance coil supportedwith said permanent magnet and oriented to produce a field to combinewith saidvfixed field therefrom, a switch for selecting a tap on saidinductance coil, circuit means adapted to be coupled to the line sweepsystemV and to said switch and said inductance coil to provide saw toothsignals at the line deflection frequency for energizing said inductancecoil and controlling the amount of deflection of said one beam inaccordance with line deflection thereof. y

4. In a color television receiver including a tri-beam cathode ray tubeadapted to produce three electron beams which travel through a neck ofthe tube and including a line and field deilecting system for developingsawtooth signals and a deflection yoke energized thereby tosimultaneously scan the beams so that they produce red, blue and greenrasters and wherein the one beam associated'with the blue raster travelsa path intermediate the paths ofthe other beams with respect to thedirection of line deflection of the beams, apparatus for dy-` namicallycontrolling the size of the blue raster including in combination, aframeY adapted to be supportedon4 the neck of the; cathode rayy tube,nductor means ,sup-b ported by said frame adjacent a predeflection pathof one beam andoriented so that a 4field produced thereby provides shiftof said one beam in the direction of line deflection, and means adaptedto be coupled to the lineV deflection Ysystem and connecting saidnductor means andV a portion of the deflection yoke in series forapplying sawtooth signals ofr line deflection frequency toksaidlinductor means for controlling said one beam in accordance thedeflected position' thereof. v

5. `In a color television receiver including Va tri-,beam cathode raytube adapted to produce electron beams which travel through a neck ofthe tube and including line andy field deflecting systems coupledV to av deflection yoke for applying sawtooth signals to said yoke for.siniultneously scanning the beams so that they produce red, blue andgreen rasters and whereinI the one beam associated with the blue rastertravels a given path intermediate the paths' of the other beams withrespect to the direction of line' deflection of the beams, apparatus fordynamically controlling the size of the blue raster with respect to thesizes of the other rasters including in combination, a frame adapted tobe Supported around the` neck of the cathode rayl tube, nductor meanssupported by said frame'adjacent a predeflection path of A said one beamandv oriented so that a field produced thereby provides shift of saidone beam in the direction of'li'edeectin, variable means'fo adjustingthe strength and sense of the field produced by sai'd nductor means,andy means for coupling the line deflection system to said induetormeans for applying sawtooth signals of linedeflection frequency' to'saidnductor means for con-k trolling said one beamVV in accordance with thedeflected' position' thereof. A l l 62 'In' a color television receiver,a tri-beam lcathode ray tube, first circuit means providing verticaldeflection signals for scanning the beams in said.y cathode ray tube,second circuit means providing horizontal deflection signals foi"scanning the beams' in'Isaid cathode ray tube, a yoke device mounted onysaid' cathode ray tube and coupled to' said first and second circuitmeans for producing a composite field for simultaneously scanning thebeams, fieldv producing means coupled' to said second circuit means tobe energized by substantially sawtooth deflection' signals' therefrom,said field producing meansl being mountedV on said cathode ray tube inyproximity to7 the'path of one' beam for modifying the horizontalideflection'of saidv` one beam independently of the horizonta'lVdeflection of the other beams, and variable means' for regulating thesawtooth` deflection signals applied toI saidj field producing means.

7. Inl al color television receiver, a tri-beam cathode? ray' tubehaving a screen with horizontal and vertlcal' tion signals for scanningthe beams vertically acr'os'ssaid sc'reen, s'econdcircuitmeans prov1d1ngsawtooth deflectionJy vsignals lfor scanning theI beams horizontally'acrosssaid* screen, ayoke' device mounted on said-h catli-Y odeA raytube and coupled to saidy first and second cir`-" cuitV means" forproducing a compositev field for s'iniultel neously" scanning the beams,said paths'y of said" beams being spaced in the horizontal' direction ofsaidi screen` with one beam path intermediate the other two;l and"variableffield producing means series coupled; with' said second circuitmeans'and said yoke device'to beV energized by'sa'wtooth signalsfrom'said. second circuit means,'said` fieldproducing means; being`mounted on saidl cathode ray" tubeinpr'oXimityt'o the path" of said onebeam1 for` modifyingi tl'ie horizontal deflection of said one beamAindependently of4 the horizontal deflection ofv the otherA beams;

8; In' a'l colortelevision receiver" including a tri-beam cathode'raytube and a line and field sweep system ,providiungglsawtooth deflectionsignals, thev beam control system including in combination, a deflectionyoke device adapted to be mounted on the cathode ray tube and to becoupled to the sweep system, said yoke device including a pair of linedeection windings and variable impedance means for apportioning signalstherebetween, a beam convergence system adapted to be coupled to thesweep system and having field producing means adapted to be mountedadjacent the predeection paths of the beams for converging the beams, abeam positioning magnet adapted to be mounted adjacent the path of onebeam for modifying the preconvergence path of said one beam in thedirection of line deflection, and variable inductor means associatedwith said beam positioning magnet and connected in series with said linedeflection windings and said variable impedance means to be energized bysawtooth signals of line deection frequency from the sweep system.

References Cited in the le of this patent UNITED STATES PATENTS2,627,052 Helpert et al. Jan. 27, 1953 2,707,248 Goodrich Apr. 26, 19552,743,381 Dietch Apr. 24, 1956 2,752,520 Morrell June 26, 1956 2,769,110Obert Oct. 30, 1956 2,780,749 Dietch Feb. 5, 1957 2,806,164* Clay et alSept. 10, 1957

